Currently, an ever increasing need for careful management of energy exists in the electric/electronic, transportation and other fields. For more precise energy control, the system is equipped with more than ever electronic components or parts. In the transportation field, for example, a transition takes place from gasoline vehicles to hybrid vehicles, plug-in hybrid vehicles, and electric vehicles. These hybrid and electric vehicles must be loaded with motors, inverters, batteries, and other electronic components or parts which are unnecessary for gasoline vehicles. Also in the body-related mechanisms such as engine control, power train, and air conditioner control mechanisms, control functions become stricter and more control systems are necessary. Accordingly, the number of electronic control units (ECU) is increasing every year. The number of electronic components loaded in such ECU units is also increasing. Nowadays, heat-conductive silicone grease compositions become indispensable to remove heat from these heat-generating electronic components or parts and conduct heat to cooling units in an efficient way.
More than ever electronic components or parts must be mounted within a limited space, indicating that electronic components or parts are kept under widely varying conditions (including temperature, mount angle, etc.). For example, heat-generating electronic components or parts and heatsinks are not held horizontal and accordingly, a heat-conductive material connecting them is often mounted at a certain angle. In such a service environment, a heat-conductive silicone adhesive material, heat-conductive potting material, or RTV heat-conductive silicone rubber composition is used in order to prevent the heat-conductive material from sagging and falling out of the space between the heat-generating component and the heatsink (as disclosed in Patent Documents 1 to 5: JP-A H08-208993, JP-A 561-157569, JP-A 2004-352947, JP 3543663, and JP 4255287). However, since all these heat-conductive materials form a complete bond to members, they undesirably lack re-workability. Since the heat-conductive material becomes very hard after bonding, it cannot withstand repeated stresses induced by thermal strain and separates apart from the heat-generating component, leading to a ramp of thermal resistance. On curing, the heat-conductive material can cause stresses to the electronic component or part.
The above problem can be solved by a one package addition cure heat-conductive silicone composition (as disclosed in Patent Document 6: JP-A 2002-327116). This composition remains re-workable and anti-sagging even after heat curing. In addition, the cured composition which is a relatively soft rubber may play the role of a stress relief agent. Nevertheless, this one package addition cure heat-conductive material suffers from several problems. For example, it must be stored in a refrigerator or freezer and thawed prior to use. In applying the one package addition cure heat-conductive silicone composition, it must be heated and cooled. Then the manufacturing system using the material must be equipped with a heating/cooling oven. The heating and cooling steps take a long time, leading to a reduction of manufacturing efficiency. From the standpoint of energy efficiency, the heating step is not regarded efficient because not only the heat-conductive material, but also overall components must be heated. Additionally, there is a potential risk that if any metal cutting fluid containing an amine compound which is a cure inhibitor is left on the coating surface, the heat-conductive material becomes under-cured.
To mitigate the cumbersome handling of one package addition cure heat-conductive silicone composition including refrigeration/thaw management for storage and heating/cooling steps for application and to obviate any concern about cure inhibition, a one package addition cure heat-conductive material which has been heat crosslinked during preparation is already proposed (Patent Document 7: JP-A 2003-301189). This heat-conductive silicone grease composition has overcome the above-discussed problems, but the tradeoff is that it has too high a viscosity to coat. There are still problems that heavy loading of filler is difficult due to the high viscosity of the base polymer and the manufacture process takes a long time.
To avoid these problems, a heat-conductive silicone composition having a lower viscosity than the above-mentioned one package addition cure heat-conductive silicone compositions and anti-sagging property was developed (Patent Document 8: JP-A 2009-286856). Although the working efficiency is significantly improved, the composition is still unsatisfactory in that the viscosity is too high to automatically carry out the steps of dispensing and screen printing the composition in a short time without mottles.
Developed under the circumstances is a room temperature and moisture-thickening heat-conductive silicone grease composition of dealcoholization type which is storable at room temperature, which has a low viscosity enough to coat at the initial, and which after coating, thickens with moisture at room temperature so that it becomes anti-sagging. Since this silicone grease composition is designed to increase its viscosity (or thicken) with moisture rather than curing with moisture, it is re-workable and applies no substantial stresses to the electronic component. However, the composition was recently found to lack long-term durability, particularly at high temperature. A new room temperature and moisture-thickening heat-conductive silicone grease composition was developed to overcome the outstanding problem. Since this room temperature and moisture-thickening heat-conductive silicone grease composition has a low viscosity at the initial (prior to thickening), its shape retaining capability is so poor that it will sag or deform immediately after coating, leaving a working problem unsolved.